ESR1 and neoplasm: A previous study, which applied xenograft tumor samples as the quality control in the IHC assessment of estrogen receptor (ER) and progesterone receptor (PR), reported that the use of xenograft tumor samples as quality controls provides advantages in relation to reproducibility of biomarker, homogeneity of tissue histology, specimen processing, availability, and cost compared with archived tumor quality controls and cell line quality controls [21].